The present invention relates to a diesel engine, which includes a piston for reciprocating inside a cylinder, a cylinder head covering an end surface of the cylinder from a side opposing to a crown surface of the piston, and a fuel injector attached to the cylinder head, and the present invention particularly relates to a diesel engine, which includes a piston with a so-called reentrant cavity formed in its crown surface.
Many diesel engines, especially comparatively compact diesel engines used in vehicles, are formed with a reentrant cavity (i.e., a cavity bulging at its central portion and tapering upward at its opening portion) at a crown surface of a piston thereof. JP2010-121483A discloses such a diesel engine.
According to the diesel engine of JP2010-121483A in which the reentrant cavity is formed in the piston, when a comparatively large amount of fuel is injected from a fuel injector within a medium or high engine load range, spray of the fuel flows to a circumferential edge portion of the cavity, and a flow of the spray reversing along a wall surface of the cavity (changing its direction toward the center of the cavity) occurs. Thus, mixing of the fuel with air is stimulated. Therefore, a generation amount of soot can be expected to be reduced.
To increase such a stimulation effect of the mixing within the medium and high engine load ranges even more, it is effective to increase the penetration (penetration force) of the fuel injected from the fuel injector. With strong penetration of the spray, the speed of the spray remains high even at a far distance from the fuel injector. Therefore, the spray can be spread farther and wider within the combustion chamber. Note that one of the factors that influence the penetration of the spray is a length of a nozzle hole of the fuel injector in an axial direction thereof (nozzle hole length). If the nozzle hole length is long, even with the same injection amount of fuel, the penetration of the spray becomes stronger. Therefore, in the case of increasing the penetration as described above, a fuel injector with a long nozzle hole length may be used, for example.
However, if the penetration of the spray is increased by for the stimulation of mixing within the medium and high engine load ranges without sufficient consideration, the cooling loss within a low engine load range is increased, which may negatively influence the fuel consumption of the engine. Specifically, within the low engine load range where the fuel injection amount is small, generally the flow of the spray reversing along the wall surface of the cavity toward the center barely occurs. Therefore, even when the fuel within the spray is combusted, the flame (combustion gas) remains around a circumferential edge portion of the cavity and part of the heat generated by the combustion is absorbed by the wall surface of the cavity. Here, if the penetration of the spray is excessively increased, the flame spreads widely across the wall surface of the cavity, and a larger amount of the combustion heat is taken by the wall surface of the cavity. As a result, the cooling loss may increase.